In an ordinary blow mold device, as shown in FIG. 6, split molds 1a, 1b where a blow cavity 2 is formed in the parting face are served as a set of molds, a blow mold device 1 is composed of a plurality of sets thereof by arranging in parallel on mold-attaching plates 3, 3 by a locating pin 4 and a bolt 5 spacing with a clearance between the plural sets, and the blow mold device is attached by making the mold-attaching plates 3, 3 into contact with the opposed faces of movable platens 6, 7 inside and outside a mold clamping apparatus.
The inside movable platen 6 has a plunger 8 coupled to the back center, and an outside movable platen 7 has rods 9, 9 coupled to both sides passing through the inside movable platen 6, so that both of the aforementioned movable platens 6, 7 move simultaneously facing to each other, and the mold close of the blow mold 1 is executed in the middle between the platens, allowing the mold to open, close, and clamping by hydraulically operating the plunger 8 and rods 9, 9 simultaneously.
In such mold clamping apparatus, mold clamping force by the plunger 8 acts on the central section of the inside movable platen 6 as pressing force, while mold clamping force by the rods 9, 9 comes to acts on both sides of the outside movable platen 7 as tensile force. Consequently, the distribution of opposed mold clamping forces is differently applied on both platens, and both movable platens 6, 7 are deflected inwardly (chain line direction) and bent backward.
According to this deflection, the aforementioned mold-attaching plates 3, 3 also finish up by deflecting similarly; however, as there is a space between adjacent molds for split molds 1a, 1b, the split mold la where the mold-attaching plate 3 is attached on the outside movable platen 6 finishes up by tilting inward, and on the contrary, the split mold 1b where the mold-attaching plate 3 is attached on the outside movable platen 7 finishes up by tilting outward. The shift due to this inclination in the opposite directions increases gradually from the central section to both sides, and in the split molds 1a, 1b places at both sides, as shown enlarged in the drawing, it becomes a considerable shifting of the parting line PL, and finishes up by creating a longitudinal shoulder on the side of the blow cavity 2.
If an injection molded preform (not shown) is pinched by the neck and inserted in the blow cavity 2 of such state, stretch blow molded full in the cavity, the trace of the parting line is transferred on the middle of the side body section of a molded product therein, and left there as a considerably protruded stripe. The parting line mark on the body is undesirable for labeling or others processes, and is rejected as defective for the packaging container of contents of high added value.